Zeolite Membrane‐Based Low‐Temperature Dehydrogenation of a Liquid Organic Hydrogen Carrier: A Key Step in the Development of a Hydrogen Economy

Abstract Methylcyclohexane (MCH) dehydrogenation is an equilibrium‐limited reaction that requires high temperatures (>300 °C) for complete conversion. However, high‐temperature operation can degrade catalytic activity and produce unwanted side products. Thus, a hybrid zeolite membrane (Z) is prepared on the inner surface of a tubular support and used it as a wall in a membrane reactor (MR) configuration. Pt/C catalysts is packed diluted with quartz sand inside the Z‐coated tube and applied the MR for MCH dehydrogenation at low temperatures (190–250 °C). Z showed a remarkable H2‐permselectivity in the presence of both toluene and MCH, yielding separation factors over 350. The Z‐based MR achieved higher MCH conversion (75.3% ± 0.8% at 220 °C) than the conventional packed‐bed reactor (56.4% ± 0.3%) and the equilibrium state (53.2%), owing to the selective removal of H2 through Z. In summary, the hybrid zeolite MR enhances MCH dehydrogenation at low temperatures by overcoming thermodynamic limitations and improves the catalytic performance and product selectivity of the reaction.


Introduction
H 2 is a promising energy carrier for resolving environmental issues including climate change and air pollution. [1]H 2 can be DOI: 10.1002/advs.202403128produced by various methods, such as electrolysis, steam methane reforming, and biomass gasification. [2]Notably, H 2 is a clean energy source, producing only water when used in fuel cells or combustion engines. [3]However, the storage and transport of H 2 is economically challenging because it requires high pressures or low temperatures for compression. [4]Liquid organic hydrogen carriers (LOHCs) are chemicals that can store and transport H 2 in liquid form via the reversible hydrogenation and dehydrogenation of organic compounds, which are liquid at standard temperature and pressure. [5]Thus, LOHCs can be stored under mild conditions, and, when H 2 is needed, can be dehydrogenated to release H 2 , yielding H 2 -lean LOHCs. [6]The released H 2 is used as an energy source, and the H 2 -lean LOHCs can be hydrogenated again for further use.This enables a closed carbon cycle that minimizes energy waste. [7]ethylcyclohexane (MCH) is an attractive LOHC because of its high H 2 content (6.16 wt.%), [8] low toxicity, and relatively low boiling point (101 °C), compared to other LOHCs, such as cyclohexane and decalin). [9]However, the dehydrogenation of MCH is endothermic, requiring high temperatures (>300 °C) for complete conversion.Unfortunately, high temperatures can degrade the catalytic activity [10] and decrease the target molecule selectivity by producing unwanted side products. [7,11]9a,10b,12] As an alternative solution, the use of a membrane reactor (MR) that combines reaction and separation in one device has been proposed to improve the reaction activity and, further, the coupling of two reactions. [13]1b,14] Further, the use of the MR configuration can increase MCH conversion and purify H 2 molecules on the permeate side at low temperatures, reducing production and separation costs. [15]9b,16] Robust inorganic materials that can withstand harsh chemical and high-temperature conditions are good candidates for the H 2 -selective membrane material in MRs, for example, metals such as palladium and its alloys [17] and porous materials such as metal-organic frameworks, [18] zeolites, [19] carbon sieves, [20] and silica. [21]In particular, zeolites are promising MR wall materials owing to their well-defined porous structures, high thermal and chemical stabilities, and molecular sieving ability. [22]As an example, the deca-dodecasil 3 rhombohedral (DDR) type zeolite has pore sizes of 0.36 × 0.44 nm 2 , [23] which makes it attractive for the separation of H 2 (kinetic diameter: 0.29 nm) [24] from the larger MCH (0.60 nm [25] ) and toluene (Tol; 0.59 nm [25] ), which are present in the MCH dehydrogenation reaction.Crucially, continuous DDR zeolite membranes can be grown from a chabazite (CHA) type zeolite seed layer. [26]13a,27] Specifically, to the best of our knowledge, to date, there have been no reports of the use of zeolite membranes in the MR configuration for MCH dehydrogenation.
In this study, we explored the use of a H 2 -selective zeolite membrane as a wall in an MR for MCH dehydrogenation.Briefly, we grew a DDR zeolite film (referred to as DDR@CHA hybrid) from a CHA type zeolite seed layer deposited on the inner surface of an -alumina tubular support.26a] Further, to ensure a fair comparison, we also used all-glazed tubes, which are impermeable to all involved molecules, to evaluate the performance of a conventional packed-bed reactor.The separation performance of the prepared DDR@CHA hybrid zeolite membranes was investigated using binary mixtures of H 2 /Tol and H 2 /MCH.The effects of MRrelevant operating conditions (weight hourly space velocity, reaction temperature, and total pressure on the feed and permeate sides) on the performance and long-term stability of the MR were also investigated.Finally, the performance of the zeolite-based MR was compared with those of a packed-bed reactor and the equilibrium state, as well as those of MRs reported in the literature.

Membrane Properties of a DDR@CHA Hybrid Zeolite Membrane (Z)
DDR zeolite membranes heteroepitaxially grown from the CHA seed layers were prepared on the inner and outer surfaces of -alumina tubular supports, denoted Z and Z out , respectively.26b] Detailed information regarding the zeolite membranes and their use for MRs is given in Subsection S1 (Supporting Information).The scanning electron microscopy (SEM) images in Figure 1a,b shows that both Z and Z out were continuous on the tubular supports, and the pyramidal spike-like grains were comparable to those of pure DDR membranes. [28]26a] The X-ray diffractometry (XRD) patterns in Figure 1c confirm that Z and Z out contained a large amount of DDR zeolite and some CHA zeolite.For clarity, the (101) reflection of CHA zeolite is indicated by an arrow in Figure 1c.These results confirm the synthesis of the DDR@CHA hybrid zeolite membrane [26b] ; indeed, the presence of a minor CHA zeolite component was a result of the CHA seed layer (≈0.2-1 μm [26b] ) remaining in the final hybrid zeolite membrane (≈2-3 μm as shown in Figure 1b1-b2).The SEM and XRD analyses confirm that intact zeolite membranes comprising mostly DDR-type zeolite were formed on the inner and outer surfaces of the -alumina tubular supports.

Separation Performance of Z
26b] Nevertheless, we tested the CO 2 /CH 4 separation performance, which represents the quality of 8-membered-ring zeolite membranes at the bulk scale. [29]As shown in Figure S1 (Supporting Information), both Z and Z out had excellent membrane qualities, having CO 2 /CH 4 separation factors (SFs) at 30 °C as high as ≈529 ± 45 and 488 ± 110, respectively.As expected, these were comparable to those of the previously reported DDR@CHA membrane [26b] (see Subsection S2.1, Supporting Information for details).In addition, we assessed the capability of Z to separate H 2 from the reactants and products of the MCH dehydrogenation reaction (Figure 2a1-a2,b1-b2; for clarity, the real values of permeances and separation factors of H 2 /Tol and H 2 /MCH through Z are given in Tables S1 and S2, Supporting Information, respectively.For a better understanding, the corresponding H 2 molar fluxes and purities are summarized in Tables S1 and S2, Supporting Information as well).The H 2 -permselectivity was marked: H 2 /Tol SFs of ≈380 from 190 to 300 °C suggested that Z is a promising high-performance MR material (Figure 2a1). [30]The high H 2permselectivity resulted in H 2 purities as high as ≈99.91% on the permeate side (Figure 2a2).Furthermore, the separation performance of Z for H 2 /MCH mixtures (because MCH is also present in the reaction) was comparable to that for the H 2 /Tol mixture.Specifically, the H 2 /MCH SFs were ≈460 (Figure 2b1), resulting in a H 2 purity on the permeate side of ≈99.93% (Figure 2b2) at 190, 220, 250, 275, and 300 °C.The difference between the H 2 /MCH and H 2 /Tol SFs (i.e., slightly higher H 2 /MCH separation performance) could be attributed to the minute difference in the molecular size of MCH and Tol, ≈0.60 and 0.59 nm, respectively. [25]16j] H 2 /Tol separation was also carried out at different pressures (1-3 bar) to account for the increased pressure in the reactor (feed side).The higher pressure reduced both H 2 /Tol SFs from 346 ± 15 to 137 ± 33 (Figure S2a, Supporting Information) and H 2 purities on the permeate side from 99.90% ± 0.003% to 99.78% ± 0.03% (Figure S2b, Supporting Information).This is possible because Tol might preferentially permeate through the few non-zeolitic regions on the membrane at high pressures, despite the low defect density in Z, as evidenced by the high CO 2permselectivities (Figure S1, Supporting Information).However, the increased H 2 molar flux should be proportional to the increase in total pressure, as shown in Figure S2b (Supporting Information), which would facilitate H 2 removal from the product stream in MR configuration and effectively shift the equilibrium state to the product side.

Z-Based MR for MCH Dehydrogenation
Before demonstrating the proof-of-concept of the Z-based MR to overcome the thermodynamic limitations of the MCH dehydrogenation reaction, we confirmed the equilibrium MCH conversions by performing the reaction in a quartz tube reactor (Q) as a reference.Specifically, at a weight hourly space velocity (WHSV) of 43 mg g −1 min −1 , the MCH conversion in Q matched the calculated equilibrium values at 170-288 °C (Figure S3, Supporting Information).To investigate the potential for the enhanced mass transfer of produced H 2 to the permeate side, we used the MR configuration with Z and Z out and found that they showed comparable MCH conversions at 190, 220, and 250 °C (Figure S4, Supporting Information).However, there was a slight improvement in the MCH conversion (pronounced at low temperatures; see Figure S4, Supporting Information), which could be ascribable to the facile permeation of H 2 from the catalyst bed to the zeolite membrane on the inner surface of the tubular support. [31]he use of the same zeolite membranes but in different positions will allow for differentiating the transport rate of the fast permeation species (here, H 2 ), which is critical for determining the MR performance [15,32] (Figure S5 (Supporting Information); for clarity, the real values of permeances and separation factors of H 2 /Tol and H 2 /MCH through Z and Z out are given in Tables S3  and S4, Supporting Information, respectively.For a better understanding, the corresponding H 2 molar fluxes and purities are also summarized in Tables S3 and S4, Supporting Information).
To elucidate the effect of the zeolite membrane on MR performance, MCH dehydrogenation reactions in a G-based packedbed reactor (i.e., glazed impermeable wall in the MR configuration as a reference) and Z-based MR were investigated at various WHSVs (7.7, 17, 26, and 35 mg g −1 min −1 ), reaction temperatures (190, 205, 220, 235, 250, 260, 275, and 300 °C), and total pressures on the feed side (1, 2, and 3 bar) (Figure 3).For the G-based packed-bed reactor at 220 °C (Figure 3a1), the MCH conversion increased with the decrease in WHSV (green).Specifically, at a WHSV of 7.7 mg g −1 min −1 , the MCH conversion only differed by ≈3% from the calculated equilibrium MCH conversion, indicating the achievement of the equilibrium state.In contrast, in the Z-based MR configuration, a decrease in WHSV led to a significant increase in MCH conversion over the equilibrium curve because of the efficient removal of H 2 from the product stream, thus shifting the equilibrium toward the product side.Furthermore, the MCH conversion at 1 bar increased with the increase in reaction temperature (from 190 to 250 °C) for both cases with Z and G, as expected for an endothermic reaction.
In particular, for the G-based packed-bed reactor, MCH conversion followed the equilibrium curve at each reaction ).In all graphs, the MCH conversions and corresponding H 2 molar flow rates at equilibrium were calculated using Equation (S5) [33] (Supporting Information).In (a1), MCH dehydrogenation reactions were conducted at WHSVs of 7.7, 17, 26, and 35 mg g −1 min −1 (220 °C; redrawn as inset) and temperatures of 190, 205, 220, 235, 250, 260, 275, and 300 °C.In (b1)-(c1), the reaction was carried out at a WHSV of 7.7 mg g −1 min −1 at 190, 220, and 250 °C.Data shown here are averaged results of three independent samples and error bars (representing the corresponding standard deviations) are included for all data points.For clarity, the error bars that are embedded in the symbols (so not conspicuous) are owing to the low standard deviation values.
temperature, indicating that the residence time was sufficient to achieve equilibrium.Notably, in the Z-based MR configuration, the MCH conversion exceeded the thermodynamically limited equilibrium conversions at all temperatures, achieving high MCH conversions at moderate temperatures, with 98.3% ± 0.7% and 99.7% ± 0.1% at 250 and 260 °C, respectively, compared to the equilibrium conversions of 88.4% and 94.5% at the same respective temperatures and almost complete MCH conversions at higher temperatures (99.9% ± 0.01% and 100% ± 0.00% at 275 and 300 °C, respectively, vs 97.5% ± 0.3% and 99.7% ± 0.1% for the G-based MR configuration at the same temperatures, respectively).
As shown in Figure 3b1-c1, we performed additional MCH dehydrogenation reactions at higher total pressures on the feed side (2 and 3 bar).We considered three different reaction temperatures (190, 220, and 250 °C) as representative temperatures.In general, Le Chatelier's principle indicates that an increased total pressure in the MCH dehydrogenation reaction will favor a reverse reaction to the reactant side.Indeed, when the total pressure on the feed side increased from 1 to 3 bar at 250 °C, the MCH conversion with G decreased from 89.8% ± 1.0% to 55.2% ± 4.6% (Figure 3a1-c1).However, in the Z-based MR, an increase in the total pressure on the feed side increased the H 2 molar flow rate to the permeate side proportionally (Figure 3a2-c2) and, thus, shifted the equilibrium state to the product side, compensating for the preferred reverse reaction at high pressures (Figure 3a1-c1).Consequently, the MCH conversion in the Zbased MR did not change considerably (98.3% ± 0.7% at 1 bar vs 97.2% ± 1.7% at 3 bar at 250 °C).Furthermore, Figures S6-S8 (Supporting Information) show the molar flow rates of H 2 , Tol, and MCH in the product stream in the G-and Z-based MRs at different total pressures (1, 2, and 3 bar) and reaction temperatures (190, 220, and 250 °C).The higher MCH conversion in the Z-based MR suggests that higher molar flow rates of H 2 and Tol and lower molar flow rates of MCH were obtained compared to those in the G-based packed-bed reactor.

Long-Term Stability of the Z-Based MR for MCH Dehydrogenation
Along with the initial MR performance, we tested the long-term stability of the Z-based MR at 220, 250, and 300 °C, as the robustness of MR is critical for practical uses.As a reference, we also performed MCH dehydrogenation reactions in the Gbased packed-bed reactor at higher temperatures of 300, 350, and 400 °C (Figure 4a).As expected from the equilibrium curve for an endothermic reaction (Figure 3a1), the Tol yields reached 99.7% ± 0.03% (close to 100%) at a relatively high temperature of 300 °C.However, the Tol yields decreased as the temperature increased above 350 °C because the unwanted demethylation of Tol occurred, yielding benzene and methane [7] : Additional information is given in Subsection S2.2 (Supporting Information).
A further increase in the reaction temperature (450 °C) resulted in activating the unwanted demethylation and producing the corresponding side products considerably (Figure S9, Supporting Information).Specifically, the Tol yield decreased from ≈99.4% ± 0.2% at 300 °C to 51.9% ± 3.1% at 450 °C.Therefore, a low working temperature in the MR, as shown in Figure 3 for the Z-based MR configuration, is desirable for practical applications.
Generally, at low temperatures, catalytic MCH dehydrogenation activity decreased, and a relatively low Tol yield of ≈78.8% was obtained during long-term stability tests at 220 °C (Figure 4b).However, at moderate temperatures (250-300 °C), the Tol yield reached ≈100%, which was maintained throughout the test without noticeable catalytic deactivation or side reactions.As expected from the robust zeolite membranes, [34] the long-term stabilities of the Z out -based MR conducted at 220, 250, and 300 °C (Figures S10 and S11b, Supporting Information) were comparable to those of the Z-based MR (Figure 4; Figure S11a, Supporting Information), indicating the effective use of zeolite membranes for the MR configuration.Unless all identical parameters are considered, a direct comparison of performance is not feasible.10b,35] Instead, a Tol yield of ≈99% at a lower temperature (e.g., 250 °C as seen in Figure 4b) was highly optimal to meet both important aspects of high MCH conversion (i.e., Tol yield) and catalyst stability, making the Z-based MR industrially available. [36]Indeed, simulation works indicated that for MCH-based H 2 production scale in the range of 30-700 m 3 h −1 , the MR configuration achieved cost reduction as high as 20.3−22.9%,compared to the conventional packed-bed reactor configuration, and was economically feasible for real uses. [15]Such benefits were consistently reported with MRs for other types of reactions. [37]Furthermore, as shown in Figure S11 (Supporting Information), the molar flow rates of H 2 on the permeate and retentate sides were almost constant, indicating the stable separation ability of both Z and Z out for up to 7 d.In summary, at a moderate temperature (250 °C), the Z-based MR avoided side reactions during MCH dehydrogenation and showed good reaction performance and durability.Notably, the current study will serve as a cornerstone to provide the technical data that can be used to estimate and determine the economic feasibility of the MR configuration in real applications, [15,37] which is, in turn, highly desirable for providing valuable insights into the MR-based advancement and realization of the hydrogen economy.

Effects of the Total Pressure of the Permeate Side on MR Performance
We further investigated the separation performance of Z in vacuum mode, considering any changes in the permeance, molar flux, H 2 /Tol SF, and H 2 purity, as well as MCH dehydrogenation  , 190, 220, and 250 °C) while keeping the pressure of the feed side at 1 bar.In (b1), the MCH conversion curve at equilibrium was calculated using Equation (S5) (Supporting Information) and is marked by a gray dashed curve.Reaction condition: P Total = 1 bar, WHSV = 7.7 mg g −1 min −1 ,and flow rate of carrier gas (Ar) = 10 mL min −1 .Data shown here are averaged results of three independent samples and error bars (representing the corresponding standard deviations) are included for all data points.For clarity, the error bars that are embedded in the symbols (so not conspicuous) are owing to the low standard deviation values.
reaction performance.The enhanced permeation of H 2 through the membrane in vacuum mode led to a significant improvement in all membrane properties (i.e., H 2 /Tol SFs of Z and, accordingly, H 2 purity on the permeate side in Figure 5a1-a2) compared to those in sweep mode (Figure 2a1-a2).
For comparison, the results in Figure 2a1-a2 are included in Figure 5a1-a2 (for clarity, the real values of permeances of H 2 /Tol through Z and their separation factors in vacuum mode are tabulated in Table S5 (Supporting Information), where the information of the corresponding H 2 molar fluxes and purities is given as well).Notably, the maximum H 2 /Tol SF in vacuum mode at 190 °C was as high as ≈1105 ± 124 with a corresponding H 2 purity of ≈99.97% ± 0.01%.
Moreover, the MCH dehydrogenation reaction performance in the Z-based MR in vacuum mode led to a more effective shift in equilibrium to the product side than that in sweep mode (Figure 5b1-b2), because of the enhanced permeation rate of H 2 to the permeate side (Figure 5a1-a2).In addition, the molar flow rates of H 2 , Tol, and MCH in the product stream at 190, 220, and 250 °C are shown in Figure S12 (Supporting Information).It was noted that stable molar flow rates of all the reactants (MCH) and products (H 2 and Tol) were observed throughout the reaction up to 190 min, indicating the robustness of the MR configuration.

Evaluation of Z-Based MR Performance for MCH Dehydrogenation
A literature survey revealed that silica, [16h,j,k] glass, [16f] carbon, [38] and Pd [16l] membranes have been used in the MR configuration for MCH dehydrogenation because of their high H 2 separation capabilities.Although we would like to compare the enhanced MCH conversion in the Z-based MR (e.g., Figure 3 in this study) with others in the literature, to the best of our knowledge, an evaluation protocol for MCH dehydrogenation reaction performance in the MR configuration has not been reported yet.Therefore, we carried out an evaluation and comparison of results obtained using different MRs reported in the literature with those of our MR, Figure 6.16f,h,j-l,38]  Specifically, the MR with both the zeolite (i.e., Z in this work) and silica membranes showed significant improvements in MCH conversion, exceeding equilibrium conversion at moderate temperatures (≈220-230 and 250-260 °C), above which the endothermic MCH dehydrogenation reaction will be thermally activated (e.g., ≥300 °C[10b] ).Although MRs based on glass, carbon, and Pd membranes also shift equilibrium to the product side, the differences between the MCH conversion values obtained in the MR configuration and the corresponding equilibrium conversion values were less notable than those of the MRs with Z and silica membranes.This suggests that the MRs with Z and silica membranes outperformed those with glass, carbon, and Pd membranes in terms of enhancing the MCH conversion.Nevertheless, it was noted that the Z-based MR yielded higher MCH dehydrogenation performance at low temperatures (180-200 °C) than silica-membrane-based MR, seemingly because of the high rate of H 2 permeation through Z in this study.Although the single-component H 2 permeance through the silica membranes was almost constant from 100 to 300 °C, [16h] the H 2 permeance in the presence of Tol in the feed was considerably reduced at low temperatures because of the physical adsorption of toluene molecules on the membrane surface or inside the pores, as reported in the literature. [39]Consequently, the reaction did not shift substantially toward the forward reaction.In contrast, Z showed almost temperature-independent separation performance for H 2 /Tol mixtures (Figure 2a).Therefore, even at low reaction temperatures, the H 2 permeance through Z in the MR configuration was not hindered by Tol adsorption onto the membrane, and its high permeance ability was maintained, suggesting a positive effect of high-performance membrane separation ability on the degree of equilibrium shift in the MR configuration for MCH dehydrogenation.
In particular, the robustness of zeolites makes Z suitable for use for MCH dehydrogenation, as compared to the conventional H 2 -permselective silica membranes.For example, the pores of pure silica membranes shrink at high temperatures as a result of the thermal condensation of the Si-OH groups. [40]16d] Although high-temperature stability tests are needed for fair evaluation and comparison, these phenomena would make silica membranes less competitive for long-term use.In contrast, zeolite membranes have a well-defined crystalline structure and are purely inorganic, resulting in thermal stability up to 500 °C. [41]Therefore, thermally stable zeolite membranes are a good option for high-temperature processes such as MCH dehydrogenation.

Conclusion
In this study, we synthesized a DDR@CHA hybrid zeolite membrane using a heteroepitaxial growth method and, further, used it in MR configuration for MCH dehydrogenation.We also investigated the effect of mass transfer between the packed catalyst and the zeolite membrane in MR configuration on MCH reaction activity.The Z membrane (i.e., a DDR zeolite membrane grown from a CHA seed layer on the inner surface of the tubular support) was similar to Z out (prepared on the outer surface) in terms of membrane morphology and crystallinity.Z in MR configuration resulted in more improvement of MCH conversion than Z out , indicating the effective H 2 removal during MCH dehydrogenation.In particular, the separation performance of Z for H 2 /Tol and H 2 /MCH mixtures was remarkable, yielding the corresponding SFs as high as ≈380 and 460, respectively, at 190-300 °C.The use of Z as walls in MR configuration for MCH dehydrogenation shifted equilibrium to the product side via the selective removal of H 2 through the membranes in sweep mode, thus overcoming the thermodynamic limit.Accordingly, the MR configuration with Z (can achieve a high H 2 purity of ≈99% or higher) exceeded the MCH equilibrium conversion and purified H 2 on the permeate side.Notably, the use of Z in MR configuration resulted in considerably improved MCH conversion under all studied conditions, which is T = 190-300 °C, P = 1-3 bar, and WHSV = 7.7-35 mg g −1 min −1 .In one case, the MCH conversion in MR configuration was 75.3% ± 0.8% at 220 °C, significantly higher than that (56.4% ± 0.3%) of a conventional packed-bed reactor, with the H 2 purity of 99.7% ± 0.2% on the permeate side.
Crucially, the use of a working temperature below 300 °C avoided side product formation (here, demethylation of Tol) and, thus, increased the H 2 purity while producing only Tol as a H 2 -lean LOHC.Further, the Z-based MR configuration exhibited robust performance, suggesting its practical potential.In addition, an enhanced equilibrium shift to the product side was achieved, apparently owing to facilitated H 2 permeation through the membrane in vacuum mode.Notably, we confirmed that the Z-based MR configuration enhanced MCH dehydrogenation performance at low reaction temperatures, and this performance was comparable to those achieved using MRs that exploited other membrane materials (especially silica).Thus, the H 2 -permselective zeolite membranes are a promising option for enhancing the low-temperature catalytic activity of MCH dehydrogenation through selective H 2 separation from the product stream.As a follow-up task, we would like to use a membrane module containing multiple zeolite membranes to increase the H 2 production rate and enhance the scope for practical applications.

Figure 1 .
Figure 1.(a1)-(b1) Top and (a2)-(b2) cross-sectional view SEM images of the DDR@CHA hybrid membranes on the (a1)-(a2) inner (i.e., Z) and (b1)-(b2) outer (i.e., Z out ) surfaces of the -alumina tubular support.(c) XRD patterns of Z and Z out along with the simulated XRD patterns of allsilica DDR and CHA zeolites.In (a1)-(b1) and (a2)-(b2), scale bars above the SEM images indicate 3 μm.Thicknesses of zeolite membranes in (a2) and (b2) are denoted by yellow arrows.In (c), the XRD peaks corresponding to the (101) plane in the CHA zeolite, owing to its minor co-presence in Z and Z out , are indicated by arrows and the asterisk ( * ) indicates the XRD peak arising from the -alumina tubular support.

Figure 2 .
Figure 2. (a1)-(b1) Permeances and SFs through Z and (a2)-(b2) the corresponding molar fluxes and H 2 purities measured at (a1)-(a2) H 2 /Tol and (b1)-(b2) H 2 /MCH molar ratios of 3:1 in the mixed feed.Separation performance tests were performed with 50 mL min −1 of Ar sweep gas at different temperatures (i.e., 190, 220, 250, 275, and 300 °C) at 1 bar on the feed side.For the H 2 purity in (a2)-(b2), the Ar sweep gas was excluded from the calculation.Data shown here are averaged results of three independent samples and error bars (representing the corresponding standard deviations) are included for all data points.For clarity, the error bars that are embedded in the symbols (so not conspicuous) are owing to the low standard deviation values.

Figure 4 .
Figure 4. Molar flow rates of hydrocarbon products and Tol yields with the (a) G-based packed-bed reactor and (b) Z-based MR as a function of timeon-stream during MCH dehydrogenation reactions.In (a), the reactions at each temperature (300, 350, or 400 °C) were conducted for 48 h, and different reaction temperatures (220, 250, and 300 °C) with the same time schedule were used in (b).In (b), only Tol was detected as a hydrocarbon product.For clarification, Tol, methane, and benzene are marked in black, red, and orange, respectively.Reaction condition: P Total = 1 bar, WHSV = 7.7 mg g −1 min −1 , flow rate of carrier gas (Ar) = 10 mL min −1 , and flow rate of sweep gas (Ar) = 50 mL min −1 .

Figure 5 .
Figure 5. (a1) H 2 and Tol permeances and H 2 /Tol SFs through Z and (a2) the corresponding H 2 and Tol molar fluxes and H 2 purities measured at a H 2 /Tol molar ratio of 3:1 in the mixed feed in vacuum mode (half-filled symbols) and sweep mode (filled symbols).The results obtained in sweep mode are identical to those shown in Figure 2a1-a2.(b1) MCH conversions and (b2) the corresponding H 2 molar flow rates during MCH dehydrogenation with G-based packed-bed reactor and Z-based MR in sweep and vacuum modes at different reaction temperatures (i.e., 190, 220, and 250 °C) while keeping the pressure of the feed side at 1 bar.In (b1), the MCH conversion curve at equilibrium was calculated using Equation (S5) (Supporting Information) and is marked by a gray dashed curve.Reaction condition: P Total = 1 bar, WHSV = 7.7 mg g −1 min −1 ,and flow rate of carrier gas (Ar) = 10 mL min −1 .Data shown here are averaged results of three independent samples and error bars (representing the corresponding standard deviations) are included for all data points.For clarity, the error bars that are embedded in the symbols (so not conspicuous) are owing to the low standard deviation values.
Figure 6.MCH conversion of the zeolite MR in the current study and those of other MRs reported in the literature.[16f,h,j-l,38]For clarification, the membrane separation modes used in the MR are indicated by filled (sweep mode) and half-filled (vacuum mode) symbols.For comparison, the MCH conversion in the conventional packed-bed reactor (empty symbols) and in the equilibrium state (marked by lines) are shown.The types of membranes used for the MR configuration are indicated at the bottom.The colors of the symbols represent the reaction temperatures (blue: 180-200 °C, green: 220-230 °C, orange: 250-260 °C, and red: 350 °C).Data (square) obtained in this work are averaged results of three independent samples and error bars (representing the corresponding standard deviations) are included for all data points.For clarity, the error bars that are embedded in the symbols (so not conspicuous) are owing to the low standard deviation values.